This study, which appeared in the American Journal of Theology 10 (1906) pp. 267-285, was written by the historian and theologian Leon Arpee. It is an analysis of information found in Greek and Armenian historical and ecclesiastical sources about the Paulicians, dissident iconoclastic Christians of Western Armenia. Hated and persecuted by both the Greek and Armenian Church establishments and savaged by the Byzantine military, the Paulicians in the 9th century made common cause with the Muslims. They were a power to be reckoned with, doctrinally and militarily, and were never destroyed as an ideological current. Mystical Sufism and extremist (ghulat) Shiism were to develop and flourish in the same geographical area once controlled by the Paulicians. Deported to the Balkans, some Paulicians founded kindred movements such as the Bogomils, and may have had an influence on the development of Protestantism in Europe. The Key of Truth, the Paulicians' only surviving work of doctrine, is examined in detail. Book scanned by Google, article extracted and uploaded by Robert Bedrosian. [Note: the Key of Truth, translated into English by F. C. Conybeare, also is available at Internet Archive.